MadSci Network: Physics

Re: Why does ice form when water that is near freezing is delivered a hard blow

Date: Wed Jan 12 22:10:41 2000
Posted By: Graeme Bushell, Faculty, Chemical Engineering, The University of New South Wales
Area of science: Physics
ID: 947122967.Ph

My guess is the following:

Whenever you have a phase change in a material, it usually requires some 
kind of assistance to get started. A good example of this is superheating. 
If you heat a (clean, unscratched) cup of water in a microwave it is 
possible to heat it slightly above its boiling point. This is possible 
because there is no convenient low energy site (like a fault in a surface) 
to allow the water molecules to break away from the water structure. If you 
then add some nucleation sites (like a teaspoon of instant coffee) the 
apparently quiescent water boils, violently.

WARNING: Be very careful if trying this at home, the reaction is quite 

My theory is that something similar happens when freezing water that is 
clean enough (no nucleation sites). If you give a sharp blow to subcooled 
water (slightly below its freezing point), this creates local pressure 
gradients (sound) which may act as inhomogeneities to let the ice crystals 
start to form.

An interesting but I think unrelated phenomenon can be seen by cooling a 
bottle of beer down to a few degrees below zero. Because the beer is under 
pressure, the contents of the bottle are liquid. As soon as you open the 
bottle, however, the pressure drops and part of the beer freezes before 
your very eyes. It is quite fascinating to watch the ice crystals grow.
(This will only occur with substances like water in which the solid is less 
dense than the liquid, for most substances the solid is more dense than the 

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2000. All rights reserved.